In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced generation of electron/hole pairs, by a material (often referred to as a charge-generation material) in the electrophotographic element. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface in the exposed areas and cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Many electrophotographic elements are designed to be initially charged with a negative polarity. They contain material, known as a hole-transport agent, which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas. A positively charged toner develops the unexposed areas. Because of the wide use of negatively charging elements, many types of positively charging toners are available. Conversely, relatively few high quality negatively charging toners are available.
For some applications, however, it is desirable to develop the exposed rather than the unexposed surface areas of the element. For example, in laser printing of alphanumeric characters it is more desirable to expose the small surface area that will form visible alphanumeric toner images, rather than waste energy exposing the large background area. In order to accomplish this with available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive toner can then develop the exposed surface areas (which will have relatively negative electrostatic potential).
An electrophotographic element designed to be initially positively charged should, however, contain an electron-transport agent, i.e., a material which facilitates the migration of photogenerated electrons toward the positively charged surface. Unfortunately, many good hole-transport agents are available, but relatively few electron transport agents are known.
A number of chemical compounds having electron-transport properties are described, for example, in U.S. Pat. Nos. 4,175,960; 4,514,481; 4,474,865; 4,559,287; 4,606,861; and 4,609,602. However, many prior art compounds have one or more drawbacks.
Some prior art electron-transport agents do not perform the electron-transporting function well under certain conditions or in certain types of electrophotographic elements. Also, some such agents cause an undesirably high rate of discharge of the electrophotographic element before it is exposed (often referred to as high dark decay).
Furthermore, some prior art electron-transport compounds have limited solubility or dispersibility in coating solvents or in polymeric binders. Attempts to include them in electrophotographic elements result in crystallization which causes problems such as undesirable dark decay, as well as scatter or absorption of actinic radiation intended to pass through the charge-transport layer.
Even when sufficient amounts of electron-transport agent can be compatibly incorporated in an electrophotographic element, problems can arise. For example, U.S. Pat. No. 4,514,481 describes a number of electron-transport agents, e.g., 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide, and illustrates incorporating them in polymeric binder layers of electrophotographic elements at a concentration of 30% by weight (based on total weight of the agent and the binder) for good performance. In fact, however, the upper limit of compatibility (solubility or homogeneous dispersibility) of compounds such as 4-dicyanomethylene-2,6-diphenyl-4H-thiopyran-1,1-dioxide in many polymeric binders is about 40% by weight. At such concentration the compounds are on the edge of incompatibility. At elevated temperatures, such as the element can encounter during normal use in a copier, the compound can more easily migrate within the binder and tend to form crystalline agglomerates.
There are additional reasons to seek electron-transport agents having improved solubility or dispersibility. For example, increasing the concentration of an electron-transport agent in a polymeric layer, in the absence of phase-separation, increases the electron-transport mobility of the layer; accordingly, photogenerated electrons move through the layer at a higher velocity and traverse the layer in a shorter period of time. The higher the mobility, the shorter is the waiting period between exposure and development, and the greater is the number of copies that can be made in a given amount of time.
Unsymmetrically substituted 2,6-diaryl-4H-thiopyran-1,1-dioxide compounds that are more readily dispersible or soluble than compounds of U.S. Pat. No. 4,514,481 are disclosed in U.S. Pat. Nos. 4,968,813, 5,013,849, 5,034,293, and 5,039,585. However, there is a continuing need for electrophotographic elements containing electron-transport agents that do not cause high dark decay and also have improved solubility or dispersibility in coating solvents and improved compatibility with polymeric binders. The compounds employed as electron-transport agents in elements of the present invention are especially soluble or dispersible and can be incorporated in polymeric binder layers at concentrations exceeding 60 weight percent.